Surface phase diagrams for the Ag–Ge(111) and Au–Si(111) systems
Introduction
Surface phase diagrams provide fundamental understanding of surface and interface phenomena, and can be used to predict the effects of changing parameters such as temperature or metal coverage on the atomic arrangement of surface structures. They also provide the starting point for understanding the kinetics of phase transitions at surfaces. However, to date there have been relatively few studies of the phase diagrams for simple metals on semiconductors where the basic Gibbs phase rules have been applied; instead, ‘phase maps’ summarizing the conditions of temperature and coverage have been used. In many cases these phase maps necessarily contain errors; for instance, unless a particular phase contains sites with variable occupancy, i.e. is a ‘surface solution’, almost without exception it can only occur in combination with another, in what is called two-phase coexistence.
The thermodynamic phase diagram of a particular system is what it will achieve if held for very long times at the specified temperatures. However, kinetic effects must also be considered. The analysis of experimental results for a certain system can be complicated by the presence of metastable phases, due mostly to kinetic effects, or by incomplete phase transitions. We propose here the use of a set of curves commonly referred to in physical metallurgy as time–temperature–transformation (T–T–T) curves as a way to represent the metastable structures that may be present in surface phase systems [1]. The T–T–T curves provide information on the time that elapses, at any selected temperature, before the transformation begins and until it is finished. When an alloy is continuously and slowly cooled, most of the transformation occurs at high temperature, whereas at a fast continuous cooling rate most of the transformation occurs at low temperatures. The ‘start’ and ‘finish’ lines are usually defined as the moments when 1% of the parent phase has transformed and when 99% of the transformation has occurred respectively.
Based on the atomic arrangement of all phases present and the interphase relationships, we report here a survey of the results of all relevant experimental reports and tentative surface phase diagrams for the Ag–Ge(111) system in the submonolayer regime and the Au–Si(111) system in the supermonolayer regime.
Section snippets
Background for Au–Si (111) and Ag–Ge (111)
An overview of the literature on thin metal deposits on clean semiconductor surfaces, in particular for the Au–Si(111) and Ag–Ge(111) systems, shows a great difference in the number of studies. Although the Au–Si(111) interface is one of the most extensively investigated, for the Ag–Ge(111) system the available information is, at best, fragmentary. A complete understanding of surface phenomena in metal–semiconductor systems requires knowledge of the atomic arrangement. Recently, several reports
The Ag–Ge(111) system
When Ag is deposited on the Ge(111) native reconstruction [the Ge(111)-c(2×8) surface], the interface undergoes a (3×1) Ag, a (4×4) Ag, and eventually a Ag reconstruction at a substrate temperature of between 200 and 450°C in the submonolayer regime. For Ag coverages above 0.1 ML, a (4×4) phase is observed, initially coexisting with the c(2×8) surface. This phase was first reported to be complete at 0.27 ML [32] and later at 0.375 ML [6], [10]. Hammar et al. [33] identified for the first
The Au–Si(111) system
The groundwork for extending the phase diagram of the Au–Si(111) system to the supermonolayer regime (up to 1.5 ML) has been laid out by the work of Plass [12]. Fig. 1 shows all the results pertaining to this problem. The experimental observations mapped in Fig. 1 are used as the basis of this study. Plass [12] did not attempt to extend his phase diagram representation to the supermonolayer regime since crucial pieces of information, particularly the atomic geometry of the and (6×6)
Discussion
In this study, we have applied the long-standing principles of physical metallurgy to the study of surface phases. The use of T–T–T curves to explain kinetically constrained phases and represent the evolution of the phase transformation with a family of curves showing different percentages of completion appears to be beneficial, although more work is required to determine the details. Since the surface phases seem to obey the same general principles as their bulk counterparts, the introduction
Acknowledgements
This work was supported by the National Science Foundation on grant number DMR-9214505.
References (46)
- et al.
Surf. Sci.
(1995) - et al.
Surf. Sci.
(1998) - et al.
Surf. Sci.
(1998) - et al.
Surf. Sci.
(1998) - et al.
Surf. Sci.
(1997) - et al.
Surf. Sci.
(1996) - et al.
Surf. Sci.
(1991) - et al.
Surf. Sci.
(1991) - et al.
Ultramicroscopy
(1989) - et al.
Surf. Sci.
(1990)
Surf. Sci.
Surf. Sci.
Ultramicroscopy
J. Cryst. Growth
Appl. Surf. Sci.
Appl. Surf. Sci.
Surf. Sci.
Surf. Sci.
Surf. Sci.
Surf. Sci.
Surf. Sci.
Surf. Sci.
Surf. Sci.
Cited by (65)
Effect of silicon surface orientation on the Au droplet motion
2023, Surfaces and InterfacesFormation of triangular islands on the Ge(111)-3×3-Ag surface
2021, Surface ScienceReal-time observation of self-interstitial reactions on an atomically smooth silicon surface
2019, Surface ScienceCitation Excerpt :Recent studies showed that the incorporation of silicon atoms could be detected by analyzing the Si-Au 3D island motion at the Si(111) surface [69,70]. It was demonstrated that 3D islands are liquid at temperatures above eutectic temperature [16,68]. In our experiments, we are investigating Au-Si system at temperatures above 900 C.
Thermal evolution of Fe on Ge(1 1 1)-c(2 × 8) surface and the effect of (√3 × √3)R30° Ag-Ge buffer layer
2015, Applied Surface ScienceUmklapp induced surface band structure of Ag/Ge(111)6 × 6
2015, Surface ScienceSelf-propelled motion of Au-Si droplets on Si(111) mediated by monoatomic step dissolution
2015, Surface ScienceCitation Excerpt :The measured temperature is corrected using as calibration points the 7 × 7 → 1 × 1 surface phase transition of Si(111), occurring at 1103 K [17], and the Au–Si eutectic point, at 633 K [18] (See the Au–Si phase diagram of the bulk phases in Fig. 1a). The phase transitions of Au-adsorbed Si(111) surfaces have been widely studied by several authors by LEED, Reflection High Energy Electron Diffraction (RHEED), REM (Reflection Electron Microscopy) and LEEM [11,17,20,21,23,24]. A 2D phase diagram deduced from our experimental results and from previously published diagrams [19–22] is drawn in Fig. 1b, to guide the reader.